1. Field of the Invention
The present invention relates to fuel injectors and in particular to unit fuel injectors for cyclically injecting fuel into an internal combustion engine in variable amounts and with varying timing.
2. Description of Related Art
Unit fuel injectors generally fall into two categories. That is, there are open nozzle injectors of the type represented by Perr U.S. Pat. No. 4,721,247 and there are closed nozzle injectors of the type represented by U.S. Pat. No. 4,463,901. Open and closed nozzle injectors have different injection characteristics, and neither type can be said to be better than the other for all possible engine applications. Thus, it can be necessary for an engine or motor vehicle manufacturer to keep both types of fuel injection systems available for installation on any given engine depending solely on the particular application for which the engine will be used.
However, to develop and produce two separate fuel injection systems, knowing only one will be used, not only increases the initial investment which must be made, but also increases later piece prices, inventory and service costs. These cost factors offset any benefits that might be associated with the fact that an injector which is specifically designed to be only a closed nozzle injector or only an open nozzle injector might be able to achieve a more optimized design than a multipurpose one, e.g., a design that might be smaller, simpler, etc.
In above noted U.S. Pat. No. 5,209,403, it was first disclosed how it could be possible to construct a unit fuel injector on a modular basis so that an upper control subassembly could be interchangeably used with either an open nozzle type or closed nozzle type injector nozzle subassembly. More specifically, this capability is attained by locating the timing chamber relief valve structure wholly in the upper part of the injector above the lower, injection plunger. However, in that case, conventional injector nozzle structure was merely adapted for use with the control subassembly, as opposed to the injector, as a whole, being designed for the specific purpose of providing a unit fuel injector system having interchangeable open and closed nozzle type injector nozzle subassemblies. Thus, the design of this earlier application does not allow the fuel injector to be converted from open to closed nozzle options without affecting engine installation, and without having to accept any performance or cost compromises relative to a fuel injector which is specifically designed to be only either a closed nozzle injector or an open nozzle injector.
U.S. Pat. No. 5,209,403 also dealt with the inability of prior relief valves, such as that in the above-mentioned Perr patent, as well as that in Warlick et al. U.S. Pat. No. 4,986,472, to alter the opening stroke of the relief valve in order to control more precisely the draining of timing fluid from the timing chamber. More specifically, the parent application provides a timing chamber relief structure which enables the opening stroke of the relief valve to be adjusted so that the timing chamber draining rate can be precisely controlled independently of spring pressure. However, not taken into consideration was the fact that, on the one hand, insufficient spring pressure can lead to chattering of the valve element while, on the other hand, high valve spring loading of the valve element can lead to excessive "droop" values (i.e., the difference between the injection pressures produced by the relief valve under low speed engine operating conditions and the injection pressures produced by the relief valve under high speed engine operating conditions), meaning that either excessive high speed or insufficient low speed pressures result.